I don't know...I would think that it's fear that it may "smell and/or taste bad. I think that guys hear rumors and take them seriously instead of finding out for themselves.
On another note, it also depends on the girl...if she's got something "goin on" down there...it wouldn't be good for a guy to go down on her...so always make sure you know what's "goin on" with your girl/wifey...
